U.S. patent application number 10/539465 was filed with the patent office on 2006-11-16 for measuring mud flow velocity using pulsed neutrons.
Invention is credited to Peter Wraight.
Application Number | 20060254350 10/539465 |
Document ID | / |
Family ID | 32479833 |
Filed Date | 2006-11-16 |
United States Patent
Application |
20060254350 |
Kind Code |
A1 |
Wraight; Peter |
November 16, 2006 |
Measuring mud flow velocity using pulsed neutrons
Abstract
The invention relates to methods and apparatus for determining
downhole mud flow rates and other downhole parameters. A method for
determining a downhole parameter includes operating a pulsed
neutron generator (6), pulsing the pulsed neutron generator (6)
off, detecting a substantially unactivated drilling fluid slug at a
known distance (d) from the pulsed neutron generator (6), and
determining a time-of-flight (t) for the unactivated drilling fluid
slug to travel from the pulsed neutron generator (6) to a detection
point.
Inventors: |
Wraight; Peter; (Skillman,
NJ) |
Correspondence
Address: |
SCHLUMBERGER OILFIELD SERVICES
200 GILLINGHAM LANE
MD 200-9
SUGAR LAND
TX
77478
US
|
Family ID: |
32479833 |
Appl. No.: |
10/539465 |
Filed: |
November 21, 2003 |
PCT Filed: |
November 21, 2003 |
PCT NO: |
PCT/EP03/13142 |
371 Date: |
May 2, 2006 |
Current U.S.
Class: |
73/152.14 ;
250/254; 250/259; 73/152.04; 73/152.31 |
Current CPC
Class: |
E21B 47/085 20200501;
E21B 47/003 20200501; E21B 47/11 20200501 |
Class at
Publication: |
073/152.14 ;
073/152.04; 073/152.31; 250/254; 250/259 |
International
Class: |
G01V 5/12 20060101
G01V005/12; E21B 47/00 20060101 E21B047/00; E21B 47/10 20060101
E21B047/10; E21B 47/08 20060101 E21B047/08; E21B 49/08 20060101
E21B049/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 31, 2002 |
EP |
02293280.0 |
Claims
1. A method for determining a downhole parameter in a drilling
environment, comprising: activating, by an activation device (6),
drilling fluid flowing past the activation device; turning off the
activation device (6) for a time sufficient to create an
unactivated slug of drilling fluid; detecting the unactivated
drilling fluid slug at a known distance (d) from the activation
device (6); and determining a time-of-flight (t) for the
unactivated drilling fluid slug to travel the distance (d).
2. The method of claim 1, further comprising calculating drilling
fluid velocity from the time-of-flight (t) and the known distance
(d).
3. The method of claim 2, wherein calculating the fluid velocity
includes using a rate-of-penetration correction.
4. The method of claim 1, further comprising calculating borehole
volume over the distance (d) using a known surface volumetric flow
rate.
5. The method of claim 4, further comprising calculating a borehole
diameter from the borehole volume.
6. The method of claim 1, further comprising calculating a downhole
volumetric flow rate from the time-of-flight (t) and a known
borehole volume.
7. The method of claim 1, wherein the method is performed using a
logging-while-drilling tool.
8. The method of claim 1, wherein the fluid flowing past the
activation device is flowing toward a surface location.
9. The method of claim 1, wherein the unactivated drilling fluid
slug is detected using a gamma ray detector located in a drill
string tool the distance d from the activation device.
10. The method of claim 1 wherein the distance d is chosen such
that the unactivated drilling fluid slug is detected within about
30 seconds from when it passes the activation device.
11. A tool for determining a downhole parameter in a drilling
environment, wherein the tool is adapted to be placed in a drill
string and wherein the tool comprises a activation device (6) and a
gamma ray detector (7) separated along a drill string axis thereof
by a distance d, the tool further comprising: control circuitry to
turn off the activation device (6) for a time sufficient to create
an unactivated slug of drilling fluid flowing past the tool; and
processing means (17), coupled to the gamma ray detector (7), for
determining when the unactivated slug of drilling fluid flows past
the gamma ray detector (7).
12. The tool of claim 11, wherein the processing means further
determines a time-of-flight (t) for the unactivated drilling fluid
slug to travel the distance (d).
13. The tool of claim 12, wherein the processing means is
configured to calculate drilling fluid velocity from the
time-of-flight (t) and the known distance (d).
14. The tool of claim 11, wherein the processing means is
configured to calculate borehole volume over the distance (d) using
a known volumetric flow rate.
15. The tool of claim 14, wherein the processing means is
configured to calculate a borehole diameter from the borehole
volume.
16. The tool of claim 12, wherein the processing means is
configured to calculate a downhole volumetric flow rate from the
time-of-flight (t) and a known borehole volume.
17. The tool of claim 11, wherein the tool comprises a
logging-while-drilling tool.
18. The tool of claim 11, wherein the fluid flowing past the
activation device is flowing outside the tool.
Description
BACKGROUND OF INVENTION
[0001] When drilling a borehole through a geologic formation, it is
important to know the downhole conditions to ensure that the drill
bit is operating correctly. These conditions include, among others,
the diameter of the borehole and, therefore, the volume of the
drilling fluid at any given point. In addition, the formation
properties are measured to predict the presence of oil or gas.
Formation properties may be logged with wireline tools, logging
while drilling (LWD) tools, or measurement while drilling (MWD)
tools. Modern oil and gas explorations typically use LWD or MWD
tools, instead of wireline tools, for formation logging due to the
saving in time and costs.
[0002] Various LWD and MWD tools are in use for measuring borehole
or formation properties. For example, LWD neutron or gamma
spectroscopy logs are used to provide lithology, formation
porosity, and formation density information. Neutron/gamma
spectroscopy is often performed by sending a pulse of neutrons into
the formation using a pulsed neutron generator (PNG). The neutrons
interact with elements in the formation by inelastic interactions
or elastic interactions. The high-energy neutrons gradually lose
their energy through these interactions to become thermal neutrons,
which may be captured by the nuclei of various elements in the
formation. After neutron capture, these elements become activated.
The activated elements then decay by emitting gamma rays. The gamma
rays emitted by these activated elements may be detected with gamma
ray detectors. Because different elements produce gamma rays of
different energies, the captured gamma ray spectra may be used to
derive the elemental compositions of the formation. The elemental
yields in turn may be used to provide formation lithology because
different sediment layers are typically enriched with different
types of elements. Methods for neutron and gamma ray logging are
well known in the art. Detailed descriptions may be found in, for
example, U.S. Pat. No. 5,440,118 issued to Roscoe, U.S. Pat. No.
5,786,595 issued to Herron et al., and U.S. Pat. No. 5,539,225
issued to Loomis et al. See also Albertin et al., "The many facets
of pulsed neutron cased-hole logging," Schlumberger Oilfield
Review, v. 8, no. 2, p. 2841, 1996.
[0003] However, various LWD or MWD tools used in formation logging
are adversely affected by the presence of drilling fluids (muds)
and their sensitivities are typically compromised by tool "stand
offs," i.e., the distances from tools (or sensors) to the borehole
wall. For example, chloride ions in the drilling muds may interact
with (capture) thermal neutrons with high efficiency reducing the
sensitivity of the gamma spectroscopy. Therefore, LWD measurements
often need to be corrected for the adverse effects from the
drilling fluids or tool stand offs. To correct the effects of the
drilling muds or tool stand offs, it is necessary to determine the
borehole diameters, tool stand offs, or the mud hold up volumes at
the sites of measurements while the borehole is being drilled.
[0004] Borehole diameters are typically measured using caliper
tools. Various caliper tools are available in the art. However,
most of these tools are useful only as wireline tools; they cannot
be deployed while drilling. With wireline tools, these measurements
are acquired after the drill strings have been pulled from the
boreholes. There would be substantial time lags between the times
when the boreholes are drilled and the formations are logged and
when the borehole diameters are determined. During this period, the
shapes and sizes of the boreholes might have changed due to
borehole instabilities. For this reason, it is desirable that the
borehole diameters are measured while the formations are logged
during the drilling process. It is also desirable that the
processes of determining the borehole diameter not interfere with
the normal logging while drilling processes.
[0005] Furthermore, large quantities of drilling fluids are pumped
through the drill strings into the boreholes while the boreholes
are being drilled. The drilling fluids help cool the cutting
surfaces of the drill bits and help carry out the earth cuttings
from the bottom of the borehole when they flow up the annulus to
the surface. To prevent formation fluids from flowing into the
borehole during the drilling process, the drilling fluids are
pumped under a pressure that is slightly higher than the expected
formation pressure. The higher hydraulic pressure of the drilling
fluids may result in a substantial loss of fluid into the formation
when a permeable and low pressure zone of the earth formation is
encountered. Detection of such fluid loss may be used in correction
of the measurements of various LWD sensors. Fluid loss into the
formation may be detected by the reduced flow back of the drilling
fluids on the surface. However, for determining in what zone the
fluid loss is occurring, means of detecting volumetric flows along
the axial depth of the borehole are needed.
[0006] Time-of-flight measurement of activated slugs of fluid have
been used in the prior art in connection with the Water Flow Log
(WFL). In the WFL service, a slug of mud is activated and then
timed over a relatively long duration. In this process, the PNG is
normally off, and is activated only very briefly to periodically
tag a slug of fluid with a neutron burst. Such a process does not
match well with the LWD environment or with neutron tools, where
the PNG remains activated most of the time.
[0007] Therefore, it would be desirable to have LWD-compatible
methods and apparatus for determining fluid time-of-flight,
borehole diameter, volumetric flow rate, and various other
parameters at a given depth in the borehole.
SUMMARY OF INVENTION
[0008] One aspect of the invention relates to methods for
determining downhole parameters. A method for determining a
downhole parameter in a drilling environment in accordance with
embodiments of the invention includes: operating a pulsed neutron
generator (6) to activate drilling fluid flowing past the neutron
generator; turning off the pulsed neutron generator (6) for a time
sufficient to create an unactivated slug of drilling fluid;
detecting the unactivated drilling fluid slug at a known distance
(d) from the pulsed neutron generator (6); and determining a
time-of-flight (t) for the unactivated drilling fluid slug to
travel the distance (d). In some embodiments, the method further
includes calculating drilling fluid velocity from the time-of-fight
(t) and the known distance (d). In some embodiments; the method
further includes calculating borehole volume over the distance (d)
using a known volumetric flow rate. In some embodiments, the method
further includes calculating a downhole volumetric flow rate from
the time-of-flight (t) and, a known borehole volume.
[0009] Another aspect, of the invention relates to a tool for
determining downhole parameters. A tool for determining a downhole
parameter in a drilling environment is a tool adapted to be placed
in a drill string, wherein the tool has a pulsed neutron generator
(6) and a gamma ray detector (7) separated along a drill string
axis thereof by a distance d. The tool further includes: control
circuitry operable to turn off the pulsed neutron generator (6) for
a time sufficient to create an unactivated slug of drilling fluid
flowing past the tool; and processing means (17), responsive to the
gamma ray detector (7), for determining when the unactivated slug
of drilling fluid flows past the gamma ray detector (7), and for
determining a time-of-flight (t) for the unactivated drilling fluid
slug to travel the distance (d). In some embodiments, the
processing means is configured to calculate drilling fluid velocity
from the time-of-flight (t) and the known distance (d). In some
embodiments, the processing means is configured to calculate
borehole volume over the distance (d) using a known surface
volumetric flow rate. In some embodiments, the processing means is
configured to calculate a downhole volumetric flow rate from the
time-of-flight (t) and a known borehole volume.
[0010] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0011] FIG. 1 shows an LWD tool in accordance with one embodiment
of the invention.
[0012] FIG. 2 shows a schematic diagram of circuitry of an LWD tool
in accordance with an embodiment of the invention
[0013] FIG. 3 shows a flow chart of an embodiment of a method of
the invention for determining a time-of-flight.
[0014] FIG. 4 shows a flow chart of embodiments of the invention
for determining various parameters from the time-of-flight.
DETAILED DESCRIPTION
[0015] The invention relates to methods and apparatus for
determining flow velocities of drilling fluids ("muds") in
boreholes. The invention, advantageously, may be used while
drilling a borehole. The fluid velocity permits the calculation of
other downhole parameters, such as the borehole diameter and the
volumetric flow rate of the mud.
[0016] In some embodiments, the invention relies on the activation
of oxygen in the drilling mud. In the activation process, oxygen
atoms in the drilling mud are transformed from stable atoms into
radioactive atoms by the bombardment of neutrons. When an oxygen-16
atom absorbs a neutron (neutron capture), it may emit a proton to
produce a radioactive nitrogen-16 atom. Nitrogen-16, with a
half-life of about 7.1 seconds, decays to oxygen-16 by emitting a
beta particle. The oxygen-16 that results from the beta decay of
nitrogen-16 is in an excited state, and it releases the excitation
energy by gamma ray emission. The gamma ray emission may be
detected by a gamma ray detector.
[0017] Embodiments of the present invention may be used with an LWD
neutron tool with no or minimal interference with normal operations
of the tool, i.e., they permit the PNG to be substantially
continuously operated for LWD measurements. Neutron logs typically
are used to measure the porosity of the formation. In addition,
elements in the formation may become activated after capturing
thermal neutrons. The activated elements then emit gamma rays when
they return to ground states. These gamma rays may be detected with
gamma ray detectors for deriving formation density or
lithology.
[0018] In a normal logging operation, the PNG in a neutron tool is
"on" most of the time to generate neutrons for the neutron log
measurements. In accordance with the invention, the PNG is pulsed
off for a period of time long enough to enable a slug of fluid to
pass the PNG without being activated. A gamma ray detector at a
known distance from the PNG measures a decrease in the count rate
when the unactivated slug of fluid passes the detector. As used
herein, an "unactivated slug" means a slug of fluid that passes
through the activation region near the PNG while the PNG is pulsed
off, even though the unactivated slug may be partially activated by
stray neutrons in the borehole or by the PNG when the slug passed
the PNG while flowing inside the drill pipe. The unactivated slug
has a lower radioactivity than an activated slug, so that a
decrease in gamma rays may be detected by the gamma ray
detector.
[0019] FIG. 1 shows one embodiment of an LWD tool 3 in a borehole
2. The LWD tool is part of the drill string 14. The LWD tool 3
includes, among other devices, a PNG 6 and a gamma ray detector 7
that are spaced apart by a known distance d. The PNG 6 has an
activation zone 11, within which atoms are activated by the
neutrons emitted from the PNG 6. As the drilling mud, flowing
upward in the annulus between the LWD tool 3 and the borehole wall
5, passes through the activation zone 11, oxygen in the mud is
activated. Arrows indicate the direction of mud flow. When the mud
passes near the gamma ray detector 7, the gamma rays emitted by the
activated oxygen are detected. When the PNG 6 is pulsed off, a slug
of mud will pass through the activation zone 11 without being
activated. When this unactivated slug reaches the gamma ray
detector 7, a decrease in the gamma ray count rate is detected. The
time between when the PNG 6 is pulsed off and the detection of the
decrease in the gamma ray count rate reflects the time for the
unactivated slug to travel from the PNG 6 to the gamma ray detector
7. This time is hereinafter referred to as the
"time-of-flight."
[0020] The distance d between the PNG 6 and the gamma ray detector
7 should be selected to optimize detection of the unactivated slug.
If the distance d is too short, then the detector receives a very
large contribution from activated oxygen within the tool. Although
this is measurable and repeatable, the statistical variation in the
count may make the measurement less accurate. On the other hand, if
the distance d is too large, then too much time elapses between
when the PNG is pulsed off and when the deactivated slug is
detected, thus making the detection unreliable. In general, the
distance should be chosen so that for normal flow velocities, d is
less than the distance travelled by mud in the annulus in about 30
seconds.
[0021] The gamma ray detector 7 may be any conventional detector
used in a neutron/gamma ray tool. In this case, the energy windows
of the gamma ray detector 7 are set such that gamma rays emitted by
activated oxygen are detected. Alternatively, the gamma ray
detector 7 may be a specific detector for the gamma ray emitted by
the activated oxygen. The mud velocity in the annulus may be
calculated using the time-of-flight and the known distance d
between the PNG 6 and the gamma ray detector 7. Equation 1 shows
one formula for calculating the mud velocity: V m = d t ( 1 )
##EQU1## Where d is the distance between the PNG 6 and the gamma
ray detector 7, t is the time-of-flight, and V.sub.m is the
velocity of the mud.
[0022] The mud velocity may then be used to compute other downhole
parameters. One such parameter is the diameter or volume of the
borehole. Another possible parameter that may be computed using the
mud velocity is the mud volumetric flow rate.
[0023] It should be noted that a slug of mud passing through the
activation zone 11 in the annulus may have already passed through
the activation zone 11 while flowing downward through the mud
channel (not shown) in the LWD tool 3. Typically, this should not
affect the time-of-flight measurement as described above for at
least two reasons. First, the mud channel has a much smaller flow
cross-section than that of the annulus. As a result, mud in the mud
channel travels through the activation zone 11 inside the drill
string much faster and is activated to a much smaller degree.
Second, the half-life of nitrogen-16 is about 7.1 seconds. Thus,
only one half of the radioactive nitrogen-16 will remain 7.1
seconds after activation. By the time the mud in the channel flows
to the drill bit and returns to the LWD tool through the annulus,
much of the radioactivity will have already decayed.
[0024] FIG. 2 shows a schematic representation of a portion of LWD
tool 3 of FIG. 1. As noted previously, the LWD tool includes a PNG
6 and a gamma ray detector 7 separated by a known distance "d". In
a given commercial implementation of an LWD tool, the tool will
include a variety of circuitry, in addition to various other
emitters and sensors, depending on the design of the tool. The
precise design of, for example, the control and processing
circuitry of the LWD tool is not germane to this invention, and
thus is not described in detail here. However, at a minimum, it
should be understood that the LWD tool 3 will include control
circuitry 15 configured to activate and deactivate the PNG 6 at
desired times. In addition, as shown in this example, the control
circuitry 15 may also control the gamma ray detector 7.
[0025] The output of the gamma ray detector 7 is applied to
processing circuitry, which for purposes of this example is shown
simply as processor 17. The processor 17 may perform, for example,
the calculation of mud velocity as set forth in Equation (1) above.
In addition, the processor 17 may perform various other
calculations as set forth in the embodiments below. One of ordinary
skill in the art will recognize that the processor 17 may be
dedicated to the functionality of this invention or, more likely,
may be a processor of general functionality to the tool.
[0026] Once the processor 17 has completed a desired computation,
the processor outputs the result to either a storage medium (for
later retrieval) or an output device (for transmission to the
surface via a communication channel). Various types of and
configurations for such devices exist and are known to those
skilled in the art. For the purposes of this explanation, these
devices are shown generically as output/storage 19.
[0027] FIG. 3 is a flow chart illustrating the embodiment of the
invention, described above, for determining the time-of-flight of
drilling mud in an LWD environment. First, shown at step 201, the
PNG is operating, i.e., is in a normally "on" state. Next, in step
202, the PNG is pulsed off for a period of time sufficient to allow
a slug of mud to flow through the activation zone (11 in FIG. 1)
while the PNG is off. The duration of the off pulse is selected
such that the size of the unactivated slug is sufficient to cause a
detectable decrease in the gamma ray count rate at the gamma ray
detector. In step 203, the decrease in the gamma ray count rate is
detected at a known distance from the PNG. As noted above, this may
be performed using any gamma ray detector known in the art or a
detector specific for the gamma rays emitted by the activated
oxygen. Then, in step 204, the time-of-flight for the unactivated
slug to travel from the PNG to the gamma detector is
calculated.
[0028] FIG. 4 is a flow chart illustrating use of the
time-of-flight to determine drilling parameters in accordance with
various embodiments of the invention. First, as explained in detail
above, the PNG is used to mark a slug of fluid (401), and the time
until the marked slug is detected by the gamma ray sensor is
measured (403). This is the time-of-flight (405). The
time-of-flight may then be used to determine other parameters of
interest. In one embodiment, given the known distance "d" between
the PNG and the gamma ray detector (407), Equation (1) above may be
employed (409) to determine mud or fluid slug velocity (411).
[0029] As noted above, in the wireline environment the size of the
borehole may be measured directly using, e.g., a caliper. However,
it is much more difficult to determine the borehole size while
drilling. A method according to one embodiment of the invention
enables the determination of the size of the borehole while
drilling. The mud is pumped into the drill string at a known
volumetric flow rate. Assuming that the mud is incompressible, that
there is no significant invasion of mud into the formation between
the drill bit and the gamma ray detector, that the tool volume is
known, and that the Rate of Penetration of the drill string is
either known or negligible with respect to the distance "d" (413),
the volume of the borehole 2 over the distance "d" can be
calculated from the time-of-flight. Specifically, the flow volume
in the annulus of the borehole over the distance d may be
calculated by multiplying the volumetric flow rate (Q) by the
time-of-flight (t). The known volume of the LWD tool 3 over the
distance "d" may then be added to the flow volume (415) to
determine the volume of the borehole (V.sub.bh) over the distance
"d" (417). Equation 2 shows this relationship: V.sub.bh=(Qt)+Vt (2)
where V.sub.tool is the volume of the LWD tool over the distance
"d", Q is the volumetric flow rate of the mud as determined from
the pump rate at the surface, and t is the time-of-flight.
[0030] The volume of the borehole V.sub.bh may, for example, be
used to calculate the average borehole diameter D.sub.bh over the
distance "d". The equation for the volume of a cylinder can be
solved for the diameter of the cylinder, as in Equation 3: D bh = 2
.times. V bh .pi. .times. .times. d ( 3 ) ##EQU2##
[0031] Some LWD tools may include sensors designed to directly
measure the diameter of a borehole during the drilling process. One
example of such a sensor is an ultrasonic sensor that determines
the diameter of the borehole by measuring the time it takes an
ultrasonic pulse to travel through the mud from the LWD tool,
reflect off the borehole wall, and return to the LWD tool. If such
a sensor is included in an LWD tool, the borehole volume over the
distance "d" may be calculated from the diameter. An embodiment of
the invention may then be used to make a downhole measurement of
the volumetric flow rate of the mud in the annulus. Specifically,
assuming the borehole volume is known over the distance "d", that
the tool volume is known, and that the ROP is either known or
negligibly small with respect to the distance "d" (419), from
Equation 2 one may determine the volumetric flow rate of the mud,
as shown in Equation (421): Q dh = V bh - V tool t ( 4 ) ##EQU3##
where t is the time-of-flight, V.sub.bh is the volume of the
borehole over the distance "d", V.sub.tool is the volume of the LWD
tool over the distance "d", and Q.sub.dh is the volumetric flow
rate of the mud in the region between the PNG and the gamma ray
detector. Although the volumetric flow rate of the mud is known at
the surface, the sub-surface measurement is useful as it provides
an indication of fluid loss into the formation (423).
[0032] The above-described equations assume that the
rate-of-penetration (ROP) of the drill bit is negligible compared
to the distance "d". In most circumstances, this assumption will
provide good results. Nonetheless, as noted above, the methods of
the invention may be adapted to take into account the
rate-of-penetration of the drill bit in those cases where it cannot
be ignored.
[0033] The ROP can be accounted for by reducing the distance
between the PNG and the gamma ray detector by the distance traveled
by the drill string during the time-of-flight measurement. The
distance traveled by the drill string is equal to the ROP times the
time-of-flight. Thus, Equation 1 can be rewritten to account for
the ROP: V m = d - ( ROP t ) t ( 5 ) ##EQU4## where ROP is the rate
of penetration, d is the distance between the PNG and the gamma ray
detector, t is the time-of-flight, and V.sub.m is the mud flow
velocity. Likewise, Equations 2-4 can be adapted to account for the
ROP by replacing d with the distance d-(ROP.times.t).
[0034] A method according to the invention could also be used in
the downward direction, i.e., while the mud is traveling down the
drill string. As described earlier, the mud in the mud channel is
activated when it passes through the activation zone 11 near the
PNG 6. The PNG 6 may be pulsed off, and the resulting decrease in
activation may be detected by a gamma ray detector (not shown)
disposed below the PNG 6 in the LWD tool 3. Although, in this
embodiment, a gamma ray detector would have to be placed below the
PNG in the drill string, the apparatus and methods of the invention
described above would not be otherwise changed.
[0035] Detection of the time-of-flight of the mud in the drill
string may be used to calibrate mud properties under downhole
conditions. For example, because the inside volume of the mud
channel is known, the time of flight may be used to derive the mud
compressibility under downhole conditions. Thus, the above
calculation of mud velocity may use this experimentally-determined
mud compressibility instead of assuming that the mud is not
compressible.
[0036] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. For example, although activation using a PNG
has been described for purposes of illustration, any activation
device would be usable within the scope of the invention.
Accordingly, the scope of the invention should be limited only by
the attached claims.
* * * * *